The move came amid increasing pressure on dissidents, in a reflection of the authorities' anxiety ahead of the sensitive date. Hundreds died as the army forced its way through Beijing to clear away demonstrators from the capital's political heart in June 1989, but the issue is taboo on the mainland.

The photo-sharing site Flickr, email service Hotmail and other services were also unavailable this evening.

"Twitter is a tool which can put all the sensitive things and sensitive guys together, very quickly. That's the very thing that the Chinese government doesn't want to see in China," said one blogger, Michael Anti, who had predicted Twitter would not be allowed for long.

"They needed time to figure out what it is and whether it needed to be controlled."

He added: "I don't know whether they will reopen Twitter after 4 June. I hope they will, for Twitter is a crucial icon for the new internet era on which many innovations emerge. China can't block their young generation from the future."

While most Chinese internet users rely on domestic services, which are heavily monitored and controlled, Twitter had become hugely popular among an urban elite. They used the site to share information on sensitive issues in recent months, such as the fire at the Chinese state television complex.

But while people could not access the site this evening, some users were still able to tweet, sending their complaints about the ban.

"We netizens were beaten by a 'combination blow'. So many famous websites are not accessible now ... This time, it is huge," wrote user williamlong.

Reuters reported that the email service Hotmail was also blocked across the mainland, while some internet users said they were unable to access Microsoft's Windows Live.

Blogger.com was blocked last month and YouTube has been inaccessible from the mainland since March.

Internet monitors have also shut down message boards on more than 6,000 websites affiliated with colleges and universities, according to the Hong Kong-based Information Centre for Human Rights and Democracy.

In a statement distributed by the same organisation, the exiled former student leader Chai Ling appealed for the release of political prisoners, an independent investigation into the events and permission for former student leaders to return home.

"The current generation of leaders who bear no responsibility should have the courage to overturn the verdicts [on the protests]," said Chai, who now lives in the US and has not commented on the issue for several years.

"The party and the government long ago reached a conclusion about the political incident that took place at the end of the 1980s and related issues," spokesman Qin Gang said when asked about the issue at the Foreign Ministry's regular news conference. The Chinese authorities deemed the protests counterrevolutionary riots.

In Taizhou, Zhejiang, officials have detained a former prisoner who last week co-signed an open letter to the government complaining about economic discrimination against dissidents, according to US-based group Human Rights in China.

Wu Gaoxing and four other men who were jailed for supporting the 1989 pro-democracy protests said former prisoners were struggling to survive because they could not find steady jobs and are deprived of medical benefits and pensions.

Calls to Taizhou's state security bureau rang unanswered.

Another signatory, Mao Guoliang, told the Associated Press: "I expect he's being held under some form of house arrest, but I don't know where."